The almost universally used process for the manufacture of alumina is the Bayer process. In a typical commercial Bayer process, raw bauxite ore is pulverized to a finely divided state. The pulverized ore is then fed to a slurry mixer where a slurry is prepared using water, spent liquor and added caustic. This bauxite slurry is then diluted and sent through a series of digesters where, at about 300.degree.-800.degree. F. and 100-2000 p.s.i., most of the total available alumina is extracted from the ore which may contain both trihydrate and monohydrate forms of alumina. The effluent from the digesters passes through a series of flash tanks wherein heat and condensate are recovered as the digested slurry is cooled to about 230.degree. F. and brought to atmospheric pressure. The aluminate liquor leaving the flashing operation (blow-off discharge) contains about 1-20% solids, which consist of the insoluble residues that remain after reaction between the bauxite ore and basic material used to digest the ore and the insoluble products which precipitate during digestion. Herein, all percentages are by weight, based on total weight, unless otherwise stated. The coarser solid particles are then generally removed with a "sand trap" cyclone. To separate the finer solid particles from the liquor, the slurry is normally fed to the center well of a primary mud settler where it is treated with a flocculant such as a polyacrylamide polymer, polyacrylate polymer, hydroxamated polymer, flour and/or starch. As the mud settles, clarified sodium aluminate solution, referred to as "green" or "pregnant" liquor, overflows a weir at the top of the mud settling tank and is passed to the subsequent process steps. The settled solids ("red mud") are withdrawn as underflow from the bottom of the primary mud settler and passed through a countercurrent washing circuit, generally comprised of a series of washers, for recovery of sodium aluminate and soda. Aluminate liquor overflowing the primary settler still contains typically 50 to 200 milligrams (mg) of suspended solids per liter. This liquor is then generally further clarified by filtration to give a filtrate with less than about 10 mg suspended solids per liter of liquor.
The aluminate liquor filtrate is typically cooled and muted to a precipitator, where seeding of the liquor may take place. A series of precipitators may be used. Alumina, in relatively pure form, is then precipitated from the filtrate as alumina trihydrate crystals. The alumina trihydrate suspension, or slurry, may then be fed to a series of decanters, or classifiers, which classify the trihydrate according to particle size. Ordinarily, some of the classifier exit streams are product streams and some are seed streams. For instance, underflow from the primary classifier is typically a product stream. Overflow from the primary classifier may be a feed stream for a secondary classifier, the underflow of which may be a product stream or a seed stream, or both. Secondary classifier overflow is typically a feed stream for a tertiary classifier, or tertiary thickener, the underflow of which is usually a seed stream, and the overflow of which is generally routed back to the ore digester. The trihydrate crystals suspended in the classifier product streams are generally washed in a hydrate tank and filtered to remove soda (sodium salts e.g. Na.sub.2 O and NaOH) and other impurities. The resulting trihydrate filter cake is then dried and calcined to give alumina trihydrate product that is suitable for commercial purposes. The seed streams from the classifiers, which tend to contain smaller trihydrate crystals than the product streams, are usually routed back to the precipitators to supply seed crystals for subsequent precipitations. The remaining liquid phase or spent liquor is returned to the initial ore digestion step and employed as a digestant of additional ore after being reconstituted with additional caustic.
U.S. Pat. No. 4,614,642 discloses a method of producing alumina trihydrate in which an alumina trihydrate suspension is subjected to a separating method, which may be chosen from the group comprising decanting, cycloning, filtering, and/or centrifuging, to produce a fraction containing fine particles. The fraction containing the fine particles is then subjected to a known type of treatment, including partially dissolving the fine particles or chemical agglomeration of the fine particles, so as to reduce their number by at least 50%.
Because of the rheological characteristics of the classifier streams, centrifugation is not typically used to separate suspended trihydrate from product or seed streams. Instead, separation of the solids is generally accomplished by the use of filters and/or other classifiers. Classifiers rely on the gravitational settling of the solids to achieve separation. Polymer flocculant may be added to some streams to increase the efficiency of separation. Flocculation of the solids aids in the settling process by tending to agglomerate smaller particles into larger ones, which tend to settle faster. Flocculation also aids in the filtering process because larger agglomerates are easier to filter than smaller ones, and less likely to plug the filtering means. Dewatering aids such as those described in U.S. Pat. No. 5,451,329 may also be added to the classifier streams to reduce the water level in the filter cake.
Because the alumina is formed in a sodium hydroxide environment, it generally contains a significant amount of soda (typically 0.3% to 0.4% for product alumina) as well as other impurities. The total soda is present as leachable soda, which can be removed by washing, and nonleachable soda, which cannot normally be removed by washing because it is contained within the alumina trihydrate crystals. For most alumina uses, such as electrolytic production of aluminum metal or formation of ordinary ceramic products, the alumina is usable even with these high levels of impurities. For a number of other applications, however, these impurity levels (particularly high soda and silica levels) are unacceptable. These applications include products intended for such uses as synthetic sapphire and as translucent bodies. Also, since the soda in the alumina is not available to be recycled back into the Bayer process, it must be replaced in the process at significant additional cost.
Much effort has been devoted to producing alumina with reduced levels of impurities, including soda. In U.S. Pat. No. 4,560,541, a process is described which involves, inter alia, reacting alumina with hydrochloric acid and adding water to dissolve the aluminum-containing reaction product, then separating the solution from the insoluble impurities by such methods as centrifugation and filtration. The removal of iron-containing impurities is taught in U.S. Pat. No. 3,607,140, which process involves the separation of iron-containing alumina hydrate by running the moving stream through a liquid cyclone, centrifuge, filter, or the like. The removal of organic impurities from spent liquor by concentrating a solution to precipitate the organic residues, then filtering or centrifuging to remove the precipitates, is revealed in U.S. Pat. No. 2,981,600. In none of these three patents was a polymer flocculant utilized.
A reduction in the turbidity of Bayer process liquors containing a cationic polymer-humate complex was achieved in U.S. Pat. No. 5,133,874 by adding a second cationic polymer to flocculate the complex and separating the flocculated polymer-humate complex by filtration, centrifugation or like. The flocculation and separation take place before the alumina trihydrate is precipitated, so the polymers are removed by the separation process and are not available to flocculate the subsequently precipitated alumina trihydrate.
Polymers containing hydroxamic acid groups for flocculation of suspended solids in Bayer process streams are described in U.S. Pat. No. 4,767,540, which is hereby incorporated herein by reference. This patent does not disclose the use of centrifugation in combination With the use of these polymers.
Surprisingly, it has now been discovered that the level of soda in alumina trihydrate is lower when polymer flocculants which contain hydroxamic acid groups are used in combination with centrifugation to dewater alumina trihydrate suspensions, than when centrifugation alone is used.
The processes of the present invention are designed to reduce the level of soda and other impurities in alumina trihydrate made by the Bayer process. The improvement forming the basis of the present invention lies in the centrifugation of alumina trihydrate crystals that have been flocculated using polymers that contain hydroxamic acid groups, as compared to centrifugation of trihydrate crystals that were not flocculated with such polymers.